Voltage-gated sodium channels are heteromultimeric integral membrane proteins that are responsible for the initial phase of the action potential in most excitable cells. A variety of inherited disorders affecting skeletal muscle contraction (hyperkalemic periodic paralysis, paramyotonia congenita, K+-aggravated myotonia), cardiac excitability (congenital long QT syndrome, idiopathic ventricular fibrillation, familial conduction system disease) and certain forms of epilepsy have been associated with mutations in various human sodium channel genes. This proposal is a competing renewal of R01-NS32387 that for 8 years has funded our efforts to elucidate the molecular genetic, physiologic and pharmacologic mechanisms of human sodium "channelopathies". We have recently shifted our focus from studies of the two striated muscle sodium channel genes (SCN4A, SCN5A) to investigations of brain sodium channel genes and their role in inherited epilepsies. We propose to perform a series of carefully integrated experiments employing molecular genetic, recombinant DNA and cellular electrophysiological approaches to elucidate the molecular defects responsible for seizure disorders linked to three distinct neuronal sodium channel genes (SCN1B, SCNIA, SCN2A). In Specific Aim 1, we propose to perform molecular genetic screening in a large cohort of families segregating seizure phenotypes consistent with generalized epilepsy with febrile scizures plus (GEFS+), severe myoelonic epilepsy of infancy (SMEI) and other less well characterized disorders that may be associated with mutations in brain sodium channels. In Specific Aim 2, we plan to perform biophysical and pharmacological characterization of epilepsy-associated mutations using recombinant human neuronal sodium channels expressed heterologously in mammalian cells. Our laboratory is uniquely qualified to elucidate the molecular mechanism of SCN1A-associated epilepsy using recombinant human SCN1A, a reagent that we have recently developed. Finally in Specific Aim 3, we will elucidate the molecular mechanisms responsible for dysfunction of the human sodium channel [31 subunit in some forms of familial epilepsy. Altogether, this work is designed to establish important correlations between genotype, clinical phenotype and biophysical properties of mutant sodium channels in human epilepsies and will have important pathophysiologic and therapeutic implications for hereditary disorders of sodium channels.